The music video was months in the making: With Bowie's approval, the song's lyrics were tweaked to reflect Hadfield's return from the International Space Station on Monday aboard a Russian Soyuz craft. "Lock your Soyuz hatch and put your helmet on," Hadfield sings in the video. After showing scenes of Hadfield strumming on his guitar and gazing soulfully out the station's windows, the video winds up with a Soyuz parachuting down to its landing.

Since "Space Oddity" went up on Sunday, it's been viewed on YouTube more than 2.7 million times.

"He's brought space back, not just for Canadians but for the world," fellow Canadian astronaut Jeremy Hansen told NBC News.

Dreams of spaceHadfield, 53, began his path to stardom during his childhood on a corn farm in southern Ontario. Watching Neil Armstrong's first steps on the moon in 1969 inspired him to dream of becoming an astronaut when he was 9 years old. He started flying airplanes in his teens, and went on to become a fighter pilot in the Canadian Armed Forces. He's been an astronaut since 1992, and he flew on space shuttle missions in 1995 and 2001.

Last December, he finally got his shot at a long-term stint in space — and he definitely made the most of the experience.

Hadfield's 28-year-old son, Evan, told NBC News that his father put in several hours a day snapping pictures and sending tweets, in addition to his usual 10-hour work shift aboard the station. "When he wasn't working directly for space station maintenance, or on one of his science experiments, he was doing something with his time to benefit people down here," Evan Hadfield said.

Evan worked long hours, too, without pay. Over the past five months, he has been managing his father's social-media accounts and taking the lead in getting videos like "Space Oddity" produced. "I work about 16 hours a day, seven days a week," he said. "Last week I worked 19 hours a day. ... I read about 13,000 to 17,000 messages a day, and that's just in the morning."

"Space Oddity" was a special case, in part due to a tangle of international copyright issues. The Hadfields started working with Bowie and his team, as well as NASA and the Canadian Space Agency, even before the astronaut's launch in December. "It was definitely something we wanted to do," Evan said.

Why do it? Chris Hadfield hinted at the reasons in a different farewell-to-space video: "Who'd have thought that five months away from the planet would make you feel closer to people?" he asked. "Not closer because I miss them — just closer because seeing this [experience] this way and being able to share it through all the media that we've used has allowed me to get a direct reflection back immediately from so many people. ... It makes me feel like I'm actually with people more, that we're having a conversation. That this experience is not individual, but it's shared and it's worldwide."

Hansen said all of Hadfield's pictures, videos and tweets could be boiled down to a simple message: "We do live on a spaceship, a spaceship called Earth, and we need to work together to protect it."

The next chapterSo what's next? After Hadfield and his two Soyuz crewmates touch down in Kazakhstan, they'll be whisked away in separate directions: Russian cosmonaut Roman Romanenko will head toward Moscow, while Hadfield and NASA astronaut Tom Marshburn will be flown directly back to NASA's Johnson Space Center in Houston for medical checks, debriefings, rest and recuperation.

"We have a lot in store for these guys over a number of weeks," Hansen said. And that's not counting a single tweet.

It's hard to believe that Hadfield will be out of the social-media spotlight for long. "We've still got a lot of stuff," Evan Hadfield said. There are still lots of photos and videos from his father's spaceflight that have yet to be shared. But not even the Hadfields know how all those visions from outer space will come out, and on what timetable.

"I don't know, and I don't even want to speculate, because what if I'm wrong?" Evan said. "I hope, I really hope that people take Dad's message to heart and continue it past his return."

Update for 12:25 p.m. ET May 14: The "Space Oddity" video viewership is up to nearly the 7 million mark, and Hadfield commented on the YouTube phenomenon shortly after his landing in Kazakhstan. "I'm very happy that ... 7 million are interested. It is very interesting and historic to be in space," Reuters quoted Hadfield as saying.

"It's part of humanity to be in space," Hadfield said in Russian. "What we were feeling, what we were doing there, the music we played, this is a big part of our lives."